One-way or overrunning clutches transmit torque in one direction only and are generally used to disengage a drive connection automatically when torque is applied in the reverse direction. Overrunning clutch elements generally are contained within the annular space between two opposed, concentric races. The inner race is keyed or otherwise fixed to an input shaft, and the outer race is fixed to a hub, or sleeve, from which the drive is ultimately taken. Alternatively, the outer race may form the torque input member and the inner race may form the torque output member.
In a sprag-type overrunning clutch, a series of rocking tumblers or sprags is held together within a cage. The cage typically includes two rings and a crimped ribbon spring in the annular space between the rings. All three components of the cage are pierced to accommodate the sprags. The ribbon spring biases the sprags upright between the inner and outer races of the drive coupling. Because the radial clearance between the races is not enough for the sprags to rock all the way to their top dead center positions, the sprags jam between the races. As long as the drive is in the direction tending to keep the sprags thus jammed, torque is transmitted from the input shaft, through the inner race, and finally to the outer race. Whenever the drive is reversed, the sprags tend to lay down against the influence of the ribbon spring, and no torque is transmitted.
A second type of overrunning clutch uses rollers instead of sprags. These rollers are housed in inclined recesses formed in the annular space between the inner and outer races. When the shaft is rotated in one direction, the rollers run up the inclines and jam between the inner and outer races. If the shaft is thereafter rotated in the other direction, the rollers run down the inclines and are freed in the space between the two races, which is wider than the diameter of the rollers. To obviate backlash in the system, the rollers may be spring-loaded up the inclines.
Because the two races rotate at high relative speeds, heat and friction can quickly wear down the components of the overrunning clutch, particularly the contact surface between the inner race and the sprags or rollers. Adequate lubrication of the components of an overrunning clutch is, therefore, very important.
Various designs have been proposed for supplying lubrication to the overrunning clutch components. These include U.S. Pat. No. 4,875,564 to Leitz et al., which discloses a sprag clutch having bores provided through lateral discs flanking the cage and the sprags. This allows oil to flow through the bores and into the space between the inner and outer races.
U.S. Pat. No. 4,874,069 to Lederman discloses a roller clutch provided with an internal oil passage in the cage that is open to one side. Oil which is normally supplied from oil passages in the inner race can then be supplied to the internal oil passage in the cage. Therefore, pressurized oil can be distributed all the way through to the center of the cage.
U.S. Pat. No. 4,756,395 to Zlotek discloses a sprag-type overrunning clutch having an actuating ring in the form of a sleeve bushing. Recesses formed in the inner radial surface of the bushing trap lubricant and create a fluid drag as the surface of the inner race rotates relative to the bushing at high relative RPM. This drag tends to displace the bushing relative to the sprag retainer so as to lift the sprags from the inner race surface in the overrunning mode, thereby reducing sprag wear.
A problem with these designs is that lubrication is not readily retained close to the inner race. The centrifugal force of the rapidly spinning races tends to fling the lubrication radially outwardly, away from the inner race. Because the inner race is the primary wear surface, an overabundance of oil must constantly be supplied to adequately lubricate the inner race contact surface.